The disclosed invention relates to video imaging systems, and is more particularly directed to a video signal processing circuit which provides for restoration of line-to-line variable DC shifts in the video signal of an AC coupled video imaging system.
AC coupled video imaging systems are utilized to provide video images of a scanned object scene. The scene being "looked at" is mechanically scanned (e.g., by a scan mirror) to provide optical radiation inputs to a plurality of detector elements, each of which is a channel that provides a detector output. The detector outputs are respectively AC coupled to amplifier stages which provide amplified video signals. The amplified video signals are utilized to produce a human observable display on an output transducer such as a CRT display or are further processed. As is well known, a video frame may include one or more fields where a plurality of fields are interlaced. On a video display, each video line of a field corresponds to the output of one detector element.
With AC coupling, each video line is independent of the other video lines and each video line is forced to have an average value of zero (i.e., no DC component). As a result, an object of uniform image brightness which spans several video lines (i.e., channels) may readily be represented differently on each video line, depending on the contents of the remaining portions of the video line. Stated another way, objects or portions of objects having non-uniform image brightness may be represented with the same intensity. The result is streaking which mars the appearance of the video image, may obscure target images, limits the usefulness of further video image processing, and reduces the probability of correct target decisions.
AC coupling removes from each video line the non-zero average (i.e., the presence of a DC component) which would have been present without the AC coupling for object scenes of interest. Of course, for an object scene of uniform brightness the average for each video line may very well be zero, but such an object scene would not be of interest and would be very unlikely. Also, AC coupling removes the line-to-line correlation that is present in images of interest.
Attempting to avoid streaking due to AC coupling by utilizing DC coupling presents its own problems, such as DC drift and the use of higher mechanical scan rates. Thus, the streaking due to AC coupling has been tolerated, and AC coupled video imaging systems are widely used.
Efforts have been made to reduce or remove streaks resulting from AC coupling, including the system set forth in U.S. Pat. No. 4,121,248, issued to C. R. Coale, Jr., on Oct. 17, 1978. The streak reduction system in U.S. Pat. No. 4,121,248 reduces streaks by calculating the statistics for a particular line and utilizing such statistics to provide a non-zero average of such particular line. The system disclosed therein is complex and ignores the line-to-line correlation which is present in images of interest.
Other known DC restoration techniques typically increase the complexity of the detector apparatus and the associated circuitry, and tend to be "invasive" by requiring the disassembly and modification of previously installed AC coupled video imaging systems. For example, with known invasive DC restoration techniques, precision optical reference radiation sources must be placed in the optical path at appropriate times during each scan and must uniformly illuminate all detectors. The responses of all the detectors to the reference radiation sources must be sensed and an appropriate DC shift must be respectively determined and applied for each channel. A further consideration is that invasive DC restoration apparatus is not readily, economically or practically retrofitted to existing installed AC coupled imaging systems.